Power supply regulator circuit and semiconductor device

Abstract

A power supply regulation circuit is provided which can achieve a preferable constant current drooping characteristic without the need for a constant current control circuit on the secondary side. During a constant current operation, a switching element (1) is turned on so as to keep constant the on duty of secondary current passing through a secondary winding (110B) of a transformer (110), so that a constant current drooping characteristic is achieved. Further, at this moment, a current limit dictating the maximum value of drain current (ID) passing through the switching element (1) is changed between first and second levels during the on time of the switching element such that the drain current (ID) has a constant peak value.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a power supply regulation circuit and a semiconductor device which have a constant current drooping characteristic as an output characteristic.BACKGROUND OF THE INVENTION[0002]Conventionally, switching power supplies have been widely available as a power supply regulation circuit for a charger, each of which achieves a constant current drooping characteristic by means of a constant current control circuit provided on the secondary side. The switching power supply comprises, on the secondary side, a resistor for detecting an output current, a constant current control circuit for controlling current passing through the resistor to a constant level, and a photocoupler for transmitting the signal of the constant current control circuit to the primary side. When the output current is equal to or higher than a fixed value, the constant current control circuit is operated. According to the switching power supply having such a con...

AI Technical Summary

Benefits of technology

[0036]In consideration of the problem, it is an object of the present invention to provide a power supply regulation circuit which makes it possible to eliminate the need for a constant current control circuit, a photocoupler, and a resistor for detecting output current, reduce a change of output current changing with input voltage in a constant current region, and achieve a preferable constant current drooping characteristic.

[0060]As described above, in the conventional switching power supply, the drain current is detected, the current limit is reduced as the output voltage reduces when the drain current increases to the current limit, and thus the peak value of the drain current is reduced, so that a constant current drooping characteristic is achieved. In the power supply regulation circuit of the present invention, the switching element is turned on so as to keep the on duty of the secondary current at the fixed value, and the switching element is turned off when the element current reaches the current limit changing between the first and second levels during the on time of the switching element. Since the constant current drooping characteristic is achieved in a different manner in the power supply regulation circuit of the present invention, the on time of the switching element is not shortened, unlike the conventional switching power supply, even when the output voltage decreases in a constant current region, and the input voltage does not change the output current.

[0062]Moreover, the power supply regulation circuit of the present invention can eliminate the need for a constant current control circuit, a photocoupler, and a resistor for detecting output current and reduce a change of output current changing with input voltage in a constant current area, so that a preferable constant current drooping characteristic can be achieved. Thus the power supply regulation circuit of the present invention makes it possible to achieve a constant current drooping characteristic with low cost, the minimum number of components, and the minimum power loss. Therefore, the power supply regulation circuit of the present invention makes it possible to obtain a sufficiently accurate switching power supply for a charger with a small number of components, and achieve low cost, miniaturization, and energy conservation for the switching power supply for a charger. For this reason, the power supply regulation circuit of the present invention is useful for a charger for portable equipment including a cellular phone and a digital still camera.

[0063]According to the power supply regulation circuit of the present invention, a change in the voltage of the third winding of the transformer is detected to detect the on duty of the secondary current, so that control can be performed so as to keep the on duty of the secondary current at the fixed value with a low power loss and a few additional components while keeping insulation between the primary side and the secondary side of the transformer.

[0066]According to the power supply regulation circuit of the present invention, the output voltage is kept constant by changing the on time of the switching element while keeping the oscillation frequency constant, so that ringing of the transformer can be prevented.

[0067]A semiconductor device of the present invention can achieve space saving and low cost for the power supply regulation circuit.

Problems solved by technology

This means that overload detection moves to the heavy load side.

However, in the case where the on time Ton is short relative to the overcurrent detection delay time Td, that is, “Td / Ton” is large, it is difficult to reduce the input voltage dependence of the peak value Ip of the drain current ID passing through the switching element.

Thus the long overcurrent detection delay time Td and the short on time Ton are disadvantageous to a reduction of the input voltage dependence of the peak value Ip of the drain current ID.

Thus in the conventional switching power supply, it is quite difficult to make the output current IO constant relative to a change of the input voltage VIN and a load.

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